This invention relates generally to vehicle drive systems and, more particularly, to battery powered drive systems such as are used in battery powered electric vehicles or hybrid vehicles.
FIG. 1 illustrates a known alternating current (AC) electric drive system that is used in battery electric vehicles and also hybrid vehicles. As shown, the energy storage unit, which may be a battery, is electrically connected to the direct current (DC) link of an DC-AC traction inverter.
FIG. 2 illustrates a known hybrid drive system that includes a first battery that is coupled to the low voltage side of the boost converter and a second battery that is coupled to the high voltage side of the boost converter. During operation, this configuration may allow a high specific-energy battery to be used as the energy storage unit, where the voltage rating of the low-side energy storage unit is generally lower than the DC link of the DC-AC traction inverter.
FIG. 3 illustrates a known hybrid drive system that includes a high specific-energy battery, an ultracapacitor, and a diode that is poled to allow current flow when the ultracapacitor voltage is lower than the voltage of the battery when arranged in the configuration shown in FIG. 3. The configuration in FIG. 3 allows increased levels of power to be transmitted from the battery to the DC link through a two-channel boost converter, where each channel or phase has the same maximum power rating.
While the above described systems are effective during various driving conditions, they may be less effective when the vehicle is operated at relative low speeds as seen during typical urban driving. As a result, the performance or fuel efficiency of the vehicle may be reduced.